Corrosion-resistant pipe coupling structures

ABSTRACT

A corrosion-resistant pipe coupling made of carbon steel has on its inner surface an intermediate annular projection directed radially inward. At least an innermost part of the annular projection is formed of corrosion-resistant material such as stainless steel. The annular projection may be covered by a corrosion-resistant annular member which is secured by friction welding, electron beam welding, an adhesive or mechanical engagement. The annular member prevents corrosion of the coupling by corrosive fluids flowing in pipes.

This is a division of application Ser. No. 358,801, filed Mar. 16, 1982,U.S. Pat. No. 4,509,776.

BACKGROUND OF THE INVENTION

This invention relates to pipe fitting structures, and more particularlyto a pipe coupling structure used for piping in chemical and otherplants, oil and gas pipelines, well tubes, and like piping.

In the piping and pipelines referred to above, multiple-wall compositepipes are used for transporting fluids such as those containingcorrosive substances. A typical multiple-wall composite pipe is a dualor double-wall pipe in which the inner wall is a liner tube made of acorrosion-resistant material for effective conducting of corrosive fluidand the outer wall or pipe is designed to provide strength to withstandinternal pressures of the corrosive fluid and external forces. Forexample, there is a double-wall pipe comprising a stainless-steel innerpipe and a carbon-steel outer pipe in which the inner pipe isinterference-fitted.

There are certain limitations to the length of such multiple-wall pipesdue to restrictions in the manufacture of the pipes and to conditions inwhich the pipes are installed in site. Therefore, pipe couplings areused to obtain a desired length of piping as is well known in the art.As a matter of course, the pipe couplings are also required to have aninternal corrosion resistance as well as a mechanical strength towithstand internal pressures and external forces.

A typical pipe coupling structure known heretofore comprises a tubularcoupling member having on axially opposite parts thereof internal orfemale screw threads engaging outer or male screw threads provided onthe end parts of the two pipes to be coupled, the coupling member havingan intermediate annular ridge or projection directed radially inwardthereof to be abuttingly interposed between the opposing end surfaces ofthe two pipes. The radially inner surface of the annular projection ismade to have the same diameter as the inner diameter of the two pipes tobe coupled whereby a smooth, continuous or uninterrupted cylindricalinner surface is formed from one pipe to the other. This means that theannular projection of the coupling member is exposed to the corrosivefluid or fluids flowing in the pipes.

As a consequence it has been customary to make the coupling member of acorrosion-resistant material such as stainless steel. This gives thecoupling sufficient resistance to corrosive fluids but isdisadvantageous in that the coupling is very expensive and lackssufficient strength, thereby resulting in increased thickness and weightand in interference with other elements and devices in and along thepipe line.

SUMMARY OF THE INVENTION

The main object of this invention is to provide a pipe couplingstructure for use with a piping having a corrosion-resistant internalsurface, wherein the strength of the coupling member is sufficient towithstand internal pressures and external forces, while the couplingmember is resistant to corrosive fluids flowing in the pipe coupled bythe member.

According to this invention, briefly summarized, there is provided acorrosion-resistant pipe coupling structure comprising a tubularcoupling member having on axially opposite parts thereof internal screwthreads engaging male screw threads provided on the axially opposing endparts of two pipes to be coupled, the internal surface of each of thepipes being resistant to corrosive fluids, and an intermediate annularprojection provided on the inner surface of the coupling member anddirected radially inward thereof to be abuttingly interposed between theopposing end parts of the pipes, the structure being characterized inthat the intermediate annular projection is made of a material resistantto the corrosive fluids at least in a radially innermost part thereof.

The detailed nature, utility, and further features of this inventionwill be more clearly apparent from the following detailed descriptionwhen read in conjunction with the accompanying drawings, brieflydescribed below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a fragmental longitudinal section of a pipe coupling structureaccording to a first embodiment of this invention;

FIGS. 2 through 4 are longitudinal sections, illustrating sequentiallyhow the structure shown in FIG. 1 can be produced;

FIGS. 5 and 6 are longitudinal sections, explanatory of a method ofproducing a pipe coupling member used in a second embodiment of theinvention;

FIG. 7 is a fragmental longitudinal section of the second embodiment ofthe invention;

FIG. 8 is a view similar to FIG. 7 but showing a modification of theembodiment shown in FIG. 7;

FIGS. 9 through 13 are longitudinal sections, illustrating sequentiallythe steps by which a coupling structure according to a third embodimentof the invention is produced;

FIG. 14 is an elevation, partly in longitudinal section, of a couplingmember used in a third embodiment of the invention;

FIGS. 15 through 17 are fragmental sections showing sequentially how endparts of the pipes to be coupled is formed;

FIG. 18 are a fragmental longitudinal section of the third embodiment ofthe invention;

FIG. 19 is a fragmental longitudinal section of a coupling member of afourth embodiment of the invention;

FIG. 20 is a view explanatory of a padding overlay to form an annularprojection shown in FIG. 19;

FIG. 21 is a fragmental longitudinal section of a coupling member of afifth embodiment of the invention; and

FIG. 22 is a fragmental longitudinal section showing a known pipecoupling structure.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 22 there is illustrated a known pipe couplingstructure comprising two dual or double-wall composite pipes 1 to becoupled, and a screwed coupling member 2 having a substantiallycylindrical body. Each of the double-wall composite pipes 1 comprises anouter wall or pipe 3 of carbon steel, for example, and an inner wall orpipe 4 of stainless steel, for example. The end portion of each pipe 1is provided with outer or male screw threads 5.

The coupling member 2 has a pair of screw-threaded parts 6 with inner orfemale screw threads 7 which are in engagement with the male screwthreads 5 of the pipes 1. The coupling member 2 has an intermediateannular ridge or projection 8 directed radially inward between the screwparts 6 so as to be abuttingly interposed between the opposing endsurfaces of the pipes 1. The annular projection 8 has annular undercutportions 9 so that annular overhang portions diverging in radiallyinward directions are formed. The end surfaces of the pipes 1 are shapedcomplementarily to the undercut portions 9 of the projection 8, as shownat 10.

When the pipes 1 are screwed into the coupling member 2, the endsurfaces 10 of the pipes are brought into tight, abutting, sealingcontact with the undercut portions 9, respectively.

However, with such construction, the annular projection 8 is exposed tocorrosive fluid or fluids flowing in the pipes and subject to corrosionby the fluid or fluids, so that there arise the disadvantages mentionedhereinbefore.

FIG. 1 shows a first embodiment of the pipe coupling structure accordingto this invention. The coupling structure comprises dual or double-wallcomposite pipes 1 to be coupled, each made up of an outer pipe 3 and aninner liner pipe 4 interference-fitted in the outer pipe 3, and acoupling member 2 coupling the two pipes 1 by engagement of its femalescrew threads 7 with male screw threads 5 formed on the pipe endportions, as in the known pipe coupling structure shown in FIG. 22.

The coupling member 2 is provided with an annular intermediateprojection 8 with annular undercut portions 9 at the two sides thereof,as is known in the art. According to this invention, the intermediateprojection 8 terminates short of the inner cylindrical surfaces of thepipes 1, and, instead, an annular member 12 of a corrosion-resistancematerial, such as a stainless steel, is fixedly secured to the radiallyinner surface of the projection 8. As shown, the inner surface of theprojection 8 is tapered at 13 and the outer surface of the annularmember 12 is also tapered correspondingly. The inner surface of theannular member 12 is cylindrical and has the same diameter as the innersurfaces of the inner pipes 1, so that an uninterrupted cylindricalsurface is formed within the pipes 1 and the annular member 12. Thus theannular member 12 constitutes a part of the intermediate projection 8.

It will be understood that the inner surface of the pipe couplingstructure is lined or covered with the corrosion-resistant materialthroughout, whereby corrosive fluid, such as crude oil, flowing in thepipe coupling structure will not corrode the outer pipes 3 and couplingmember main body 2 which are made of non-corrosion-resistant material,such as a high tensile carbon steel. Moreover, strength to withstandinternal pressures and external forces is assured by the high tensilecarbon steel forming the outer pipes and the coupling member.

The pipe coupling structure of the above construction may be produced bythe following procedure.

First, as shown in FIG. 2, a cylindrical body 2a to be formed into thecoupling member 2 and a small frustoconical body 12a to be formed intothe annular member 12 are prepared separately. The cylindrical body 2ais prepared by machining a non-corrosion-resistant stock material so asto have a cylindrical outer surface, a pair of frustoconical recesses 14coaxial with the outer surface, and a tapered bore 13 coaxiallyconnecting the recesses 14 in mutual communication, with an annularshoulder 15 formed between the bore 13 and each recess 14. Thefrustoconical body 12a is prepared by machining a corrosion-resistantstock material so as to have a conical outer surface corresponding tothe tapered bore 13 and an axial bore 16.

Thereafter, a chucking head 18 secured to a rotating spindle 19 isinserted into the bore 16 of the frustoconical body 12a and the head 18is expanded for chucking. The spindle 19 is then rotated at a highspeed, and the spindle and the body 12a are advanced into the interiorof the cylindrical body 2a as indicated by arrow P. The body 12a isinserted finally into the tapered bore 13 and thrust forwardly intofrictional sliding contact with the surface of the bore 13 by means of athrusting mechanism, not shown. The surface of the tapered bore 13 andthe outer conical surface of the body 12a are therefore subjected tofriction welding due to frictional heat generated therebetween, and thefrustoconical body 12a is fixedly secured to the cylindrical body 2a asindicated in FIG. 3.

The assembly thus obtained is thereafter subjected to machining to formthe female screw thread 7 and the annular undercut portions 9 as shownin FIG. 4. Thus, the pipe coupling member 2, which is to be applied tothe pipes 1 as shown in FIG. 1, is produced.

Although the male and female screw threads 5 and 7 are illustrated asbeing tapered axially in the example shown, these screw threads need notbe tapered but may be straight.

The above described method of producing the pipe coupling member isadvantageous in that the welding surface area can be determined at willto obtain a desired strength of welding with high precision and in that,since the machining is carried out after the friction weld has beenmade, exact shaping of the undercut portions as well as clean trimmingof the edges of the friction welded surfaces can be made to attain ahigh degree of tightness or excellent seal between the pipes and thecoupling member.

In the pipe coupling structure described above, the corrosion resistantannular member 12 is secured to the coupling member 2 by frictionwelding, but the annular member can be secured to the coupling member byother securing means such as electron beam welding, laser beam weldingand the like, or by adhesives.

FIG. 5 through 6 show a second embodiment of the invention. In thesefigures, parts and elements equivalent or corresponding to those shownin FIGS. 1 through 4 are designated by the same reference characters.

In this embodiment of the invention, the main body itself of the pipecoupling member 2 is not provided with an integral intermediate annularprojection but is formed with a cylindrical securing surface 20 whichdoes not project radially inward beyond the female screw threads 7 ofthe coupling member 2, and to which a separately formed annular member21 made of a corrosion-resistant material such as stainless steel issecured.

The annular member 21 has an outer cylindrical surface 22 and an innercylindrical surface 16. The annular member 21 has also annular undercutportions 9 at opposite sides thereof so that an annular overhang portion8 diverging in a radially inward direction is formed. The outercylindrical surface 22 of the annular member 21 is of substantially thesame diameter as the securing surface 20. Therefore, when the annularmember 21 is placed in the space defined by the securing surface 20, theouter cylindrical surface 22 is in contact with the securing surface 20.After such a condition is obtained, a suitable expanding or chuckingdevice 24 is fitted in the annular member 21 as shown in FIG. 6 to pressthe cylindrical surface 22 of the annular member 21 against the securingsurface 20. Thereafter, beam welding, such as electron beam welding, iscarried out along the interface between the surfaces 20 and 22 whereuponthe annular member 21 is fixedly secured at the securing surface 20 tothe main body of the pipe coupling member. The electron beam welding iscarried out, for example, by projecting an electron beam from at leastone side of the annular interface between the coupling member 2 and theannular member 21. For this purpose, an electron beam gun is moved alongthe edge of the annular interface relative to the coupling member andthe annular member. The electron beam welding can be replaced by laserbeam welding.

When the thus produced pipe coupling member 2 is applied to the pipes 1,a pipe coupling structure as shown in FIG. 7 is obtained. In thisstructure, the annular undercut portions 9 of the annular member 21 arefluid-tightly abutted against the complementarily shaped end surfaces ofthe pipes 1, and the inner cylindrical surface 16 of the annular member21 smoothly connects the inner surfaces of the pipes as shown. It willbe noted that the non-corrosion-resistant materials of the outer pipes 3and the coupling member 2 are thus prevented from being corroded bycorrosive fluid flowing in the pipe coupling structure.

The annular member 21 may be secured to the coupling member 2 by anadhesive as indicated at 26 in FIG. 8. Epoxy industrial adhesives aresuitable for this purpose. When securing the annular member 21 to thecoupling member 2, adhesive 26 is applied to at least one of thesurfaces 20 and 22 and a suitable pressing or chucking device 24 isfitted in the annular member 21 to press the surface 22 firmly againstthe surface 20.

In a third embodiment of this invention illustrated in FIG. 18, anannular member 29 for protecting the pipe coupling member 2 fromcorrosive fluids is made from a corrosion-resistant sheet of metal, suchas stainless steel. The coupling member 2 itself, shown in FIG. 14, hasan annular projection 8 integral with the main body of the coupling,which projection 8 is drected radially inward. The annular projection 8is formed with a pair of annular undercut portions 9 at its two sides.The annular member 29 has at its two edges respective flanges 30 whichare bent radially outward so as to conform to the shape of the undercutportions 9. The annular member 29 is thus secured mechanically to theinner cylindrical surface of the projection 8. The bent flanges 30engaging the undercut portions 9 prevent the annular member 29 frommoving axially and radially inward relative to the projection 8.

The coupling member 2 shown in FIG. 14 may be produced by the stepsshown sequentially in FIGS. 9 through 13.

First, a coupling body 2 shown in FIG. 9 is prepared by machining astock material of a non-corrosion resistant material, such as a hightensile carbon steel. The coupling body 2 is formed with female screwthreads 7, the annular projection 8 with a cylindrical surface 32, andthe undercut portions 9.

A cylindrical body 29a, shown in FIG. 10 and made of acorrosion-resistant material, is prepared separately. The outer diameterof the cylindrical body 29a is the same as the diameter of thecylindrical surface 32. The cylindrical body 29a is fitted tightly intothe bore defined by the surface 32 as shown in FIG. 10.

Thereafter, as shown in FIG. 11, a press jig 34, capable of beingdivided circumferentially for disassembly, is inserted into thecylindrical body 29a for pressing the latter firmly against thecylindrical surface 32 of the projection. The press jig 34 is formedwith a central opening 35.

A tightening bolt 36 is then passed axially through the opening 35 asindicated in FIG. 12 and a pair of dies 37 each having a central axialhole are moved onto the bolt 36 from the two ends thereof with the bolt36 passing through and projecting beyond the central holes of the dies37. Each die 37 has a beveled or frustoconical annular surface 38 on itsside opposing the other die. After the dies 37 are supported by the bolt36, nuts 39 are screwed onto the two ends of the bolt and tightened. Bythe tightening of the nuts 39 the two dies 37 are forced toward eachother, whereby the edge portions of the cylindrical body 29a are bentradially outward until they conform to the beveled surfaces 38, as shownin FIG. 12.

The dies 37 are thereafter removed after removal of the nuts 39, andanother pair of dies 40 each formed with a countersunk annular surface41 extending substantially parallel to the radially inner part of theundercut portion 9 are placed on the bolt 36 as shown in FIG. 13 and thenuts 39 are again tightened as shown, whereby the once bent edgeportions of the cylindrical body 29a are further bent until they conformto the contours of the radially inner parts of the undercut portion 9.Thus, when the nuts 39, the bolt 36 and the press jig 34 are removed,the pipe coupling member 2 provided with the corrosion-resistant annularmember 29 having the bent flanges 30 is obtained.

The pipes to be coupled by the coupling member 2 as described above maybe formed as follows. Each pipe 1, comprising an outer pipe 3 and aninner corrosion-resistant pipe 4, is prepared to have the end 4a of theinner pipe projecting axially beyond the end of the outer pipe 3, asshown in FIG. 15, so as to form an annular shoulder, on which an annularpadding 43a of stainless steel is overlaid thereafter as indicated inFIG. 16. The thus prepared pipe 1 is then machined to form male screwthreads 5 on the outer surface of the outer pipe 3 and an annularundercut portion 44 at the pipe end as shown in FIG. 17. It will benoted that by undercutting the padding 43a at 44, an annularcorrosion-resistant cover 43 with an annular overhang 45 is formed.

When two such opposing pipe end portions are coupled by the couplingmember 2 shown in FIG. 14, the pipe coupling structure as shown in FIG.18 is obtained, in which the annular projection 8 is interposed betweenthe opposing annular covers 43 in sealing contact, the overhangs 45 ofthe covers 43 tightly engaging the respective undercut portions 9 of theannular projection. The inner cylindrical surface of the annular member29 forms smoothly a continuation of the inner surfaces of the innerpipes or linings 4. It is to be noted that each bent flange 30 of theannular member 29 is in close contact with the surface of the respectiveundercut portion 44 and this also assures high tightness, and that thebent flanges 30 serve to eliminate the possibility of the corrosivefluid reaching the non-corrosion resistant material of the projection 8.

Although the flanges 30 are bent in two steps as indicated in FIGS. 12and 13, it is possible to carry out the bending of the flanges 30 in onestep.

The advantage of the third embodiment of the invention is that, thoughthe annular member 29 is thin, it can be secured to the annularprojection 8 firmly and can protect the annular projection 8 fullyagainst corrosive fluids.

FIG. 19 illustrates a fourth embodiment of the invention wherein theannular projection 8 entirely made of corrosion-resistant material isalong its outer periphery embedded in an annular groove 46. In order toform the projection 8, a cylindrical main body of the coupling member 2is first formed with an annular groove 46 as illustrated in FIG. 20, andthen an annular padding 8a of stainless steel, for example, isoverlay-welded in the groove 46. Thereafter, the padding 8a is machinedinto the shape having an inner cylindrical surface 47 and a pair ofundercut portions 9.

In the fifth embodiment of the invention, the annular projection 8 hasits outer diameter equal to the outer diameter of the coupling member 2.The annular projection 8 is prepared separately, and is welded tocylindrical members 49 at 50 by electron beam welding, for example, asindicated by arrows.

Although the invention has been described in connection with a pipe madeof an outer pipe and an inner liner pipe, it can equally be applied topipes of single-wall structure entirely made of corrosion-resistantmaterial.

We claim:
 1. A corrosion-resistant metal pipe coupling structure comprising:a tubular metal coupling member including an inner surface and axially opposite ends having internal screw threads for engaging male screw threads provided on axially opposing end parts of two metal pipes to be coupled, the internal surface of each of the pipes having secured thereto an inner metal lining pipe resistant to corrosive fluids; and said coupling member including an integral intermediate annular projection extending radially inwardly from said inner surface to be abuttingly engaged by the opposing end parts of the pipes, said annular projection being formed entirely of an annular corrosion-resistant metal member embedded in an annular groove formed in the radially inner surface of said coupling member, said metal member being disposed between and separated from said internal screw threads at said opposite ends of said coupling member, said annular projection including on axially opposite sides thereof undercut portions with which the respective opposing ends parts of the two pipes are in tight sealing engagement.
 2. A structure in claim 1, wherein radially inner surfaces of said lining pipes and of said annular projection form a smooth continuous surface.
 3. A corrosion-resistant metal pipe coupling structure comprising:a tubular metal coupling member including an inner surface and axially opposite ends having internal screw threads for engaging male screw threads provided on axially opposing end parts of two metal pipes to be coupled, the internal surface of each of the pipes having secured thereto an inner metal lining pipe resistant to corrosive fluids; and said coupling member including an integral intermediate annular projection extending radially inwardly from said inner surface to be abuttingly engaged by the opposing end parts of the pipes, said annular projection being formed entirely of an annular corrosion-resistant metal member, said coupling member comprising a pair of axially spaced cylindrical members having therein said internal threads, said annular member being positioned between and fixed to said cylindrical members, said annular member being disposed between and separated from said internal screw threads at said opposite ends of said coupling member, said annular projection including on axially opposite sides thereof undercut portions with which the respective opposing end parts of the two pipes are in tight sealing engagement.
 4. A structure as claimed in claim 3, wherein the outer diameters of said annular and cylindrical members are equal.
 5. A structure as claimed in claim 3, wherein said annular member is welded to said cylindrical members.
 6. A structure as claimed in claim 3, wherein radially inner surfaces of said lining pipes and of said annular projection form a smooth continuous surface. 